In the prior art emergency lighting driver devices for driving a lighting unit with at least one emergency lighting source, in particular a LED unit with at least one LED, are well known.
In complex lighting systems, such as the lighting system in a large building, emergency lighting driver devices are vital in order to provide emergency lighting in case the normal electrical energy supply of the lighting system, such as e.g. mains, is disturbed or interrupted. That is, in case of emergency event, such an emergency lighting driver device is configured to provide an emergency lighting, wherein the emergency lighting driving device comprises a battery for driving a lighting unit comprising at least one emergency lighting source in the emergency operation mode. An emergency event is for example a black-out caused by the public utility company or a fire which disturbs or interrupts the normal electrical energy supply. The emergency lighting driver device is configured to monitor the normal electrical energy supply, in order to drive the lighting unit with at least one emergency lighting source in the emergency operation mode upon detecting a drop in the electrical energy or voltage of the normal electrical energy supply. During the normal operation mode, i.e. when the normal electrical energy supply is working, the emergency lighting driver device does not drive the lighting unit with the at least one emergency lighting source. The emergency lighting driver unit may charge its battery in the normal operation mode.
Specific emergency lighting driver devices are known which are configured to drive the lighting unit, in addition to the battery-driven emergency operation mode, in the normal operation mode, wherein the electrical energy for driving the lighting unit comes from a mains voltage.
The battery of the emergency lighting driver device must be charged during the normal operation mode with electrical energy from the normal electrical energy source, i.e. mains, in order to be able to provide electrical energy for driving the lighting unit during the emergency operation mode.
The battery of an emergency lighting driver device is typically charged during the normal operation mode. For charging the battery a pulse charge method may be used, i.e. the battery is alternately charged for a certain time period and then not charged for a certain time period. To implement such a pulse charging, the battery is alternately connected and disconnected from the supply with electrical energy starting from the normal electrical energy source, wherein the battery is charged with electrical energy when being connected to the normal electrical energy source and not charged when being disconnected.
Usually this is achieved by connecting a switch in series to the battery, wherein the series connection of the battery and the switch is in parallel to the normal electrical energy source. For performing the pulse charge method the switch is then controlled using a pulse signal, wherein e.g. the switch is turned on, i.e. switched to the conducting state, as a result of a rising edge of the pulse signal and turned off, i.e. switched to the non-conducting state, as a result of a falling edge of the pulse signal. When the switch is in the conducting state, the battery is charged with electrical energy from the normal electrical energy source and when the switch is in the non-conducting state the battery is not charged with electrical energy. The pulse charge method is preferred for charging nickel-metal hydride batteries (NiMh).
The present invention is now concerned with a further specific type of emergency lighting driver devices that are configured                to drive a lighting unit with at least one emergency lighting source with the electrical energy stored in its battery in the emergency operation mode,        to drive the lighting unit with electrical energy provided from the normal electrical energy source, e.g. mains, in the normal operation mode, and        to charge its battery with electrical energy provided from the normal electrical energy source, while driving the lighting unit with electrical energy provided from the normal electrical energy source, in the normal operation mode.        
When such an emergency lighting driver device according to the state of the art drives a LED load during the normal operation mode, while the battery of the emergency lighting driver is being pulse charged, the light emitted by the LED load may show a flicker, i.e. a visible fluctuation in the intensity of the light emitted by the LED load.
Namely, due to the connecting and disconnecting of the battery to respectively from the normal electrical energy source as a result of switching the switch in series to the battery during the pulse charging, the electrical energy, in particular the current, that is supplied to the LED load may rapidly increase, i.e. overshoot, or rapidly decrease, i.e. undershoot, causing the flicker.
Even in case a LED driver is present in the emergency lighting driver device for feedback-controlling the current flowing through the LED load, the flicker occurs, as the feedback control is typically too slow in order to react to the overshooting or undershooting caused by the connecting and disconnecting of the battery to respectively from the normal electrical energy source.
Therefore, it is an object of the present invention to provide an emergency lighting driver device that is configured to pulse charge its battery during the normal operation mode, while driving a LED load at the same time, such that the light emitted by the LED load does not show a flicker or respectively the flicker of the emitted light is reduced. In other words, it is an object of the present invention to overcome the above described problem when pulse charging the battery of an emergency lighting driver device during the normal operation mode, while a LED load is driven by the emergency lighting driver device.
This and other objects, which become apparent upon reading the following description, are solved by the invention.